Applications for metal oxide semiconductor field effect transistors ("MOSFETs") are extremely widespread. Demands for increased functionality from smaller chip areas requires that these transistors be downsized. However, as they are downsized, their performance is increasingly affected by various parasitic capacitances. Therefore, it becomes extremely important to understand both the DC and AC performance characteristics of the transistors before circuits are designed. Otherwise, circuits will not work as designed.
For example, when designing a high speed CMOS circuit, with design dimensions in the sub-micron range, an imprecise DC or AC model may result in a circuit design that simply does not work when implemented in silicon. In particular, a model that underestimates various capacitances can result in a circuit designed for an operating speed that will not meet that speed when implemented.
The designed channel length ("L.sub.des ") Of a transistor is comprised of the sum of the effective channel length ("L.sub.eff "), the gate length reduction ("LR") (caused due to polysilicon patterning of the gate), and the total lateral diffusion ("TLD") (which causes gate-drain overlap). By using these parameters, among others, fairly accurate DC models for MOSFETs can be derived, for example, with the SPICE design tool.
Techniques exist for determining L.sub.eff. However, generally only the sum of LR and TLD is obtained, which is acceptable for the DC model, since that sum is all that is necessary for an accurate DC model. In contrast, the AC performance of the transistor depends critically on the TLD, and existing techniques do not allow for an accurate and straightforward determination of TLD.
Gate-to-drain overlap (the use of this term herein includes gate-to-source overlap) caused by TLD results in increased overlap capacitance that slows down circuit speed due to what is known as the Miller Effect. In the AC operation, this effect causes either overshoots or undershoots in the waveforms that result in increased delay time for propagation.